CAPÍTULO 4. SISTEMAS DE INFORMACIÓN
4.8 B ASES DE D ATOS
Dept of Applied Geology,
Federal University of Technology, Akure.
Nigeria.
O.S. Oladeji
Dept of Civil Engineering,
Ladoke Akintola University of Technology, Ogbomoso.
Nigeria. Email: [email protected] Telephone: +2348163240876
Abstract
This work presents depth-area stratification of groundwater quality within Lagos metropolis, Nigeria. The hydrochemical data were checked for validity via ionic balance to ensure an acceptable 5 % error limit. Spatially, two major hydrogeochemical regimes, namely Na-Cl and Na-HCO3
facies types were identified in the study area. Also, depth-dependent geochemical facies transformation from Cl-SO42- type at shallow horizons, through Cl-SO4
2--HCO3
at the lower horizons, and to HCO3
--Cl-SO4
at the deep horizons were observed. Results of the algebraic-graphical analysis of the observed salinity suggest an ancient water origin entrapped by marine sediments rather than recent salt water intrusion. The pH values indicate higher (5.3) acidity in the northern part as compared with the central (5.9) and the southern (6.0) regions of the study area. The major ionic concentrations viz: Ca2+, Mg2+, Na+, K+, SO4
2-, HCO3
-, and Cl-, generally fall below the W.H.O. suggested desirable limits. The SAR limits of between 3.08 and 14.0 were observed for the water samples in the area, and this suggests excellent to good water type for irrigation purposes. Regular routine chemical analysis is recommended to monitor possible ingress of salinity into the aquifers.
KEYWORDS: Hydrochemistry; groundwater quality; hydrochemical regimes; statistical approach
40 Introduction
The success of the global efforts in the development of groundwater resources potential depends largely on the ability to adequately protect this subterranean water from the increasing threat of subsurface pollution. However, the compelling desires to reduce surface environmental pollution has caused some practitioners in the waste management fields to covet the subsurface environment as a waste disposal unit, thereby aggravating the potential for groundwater pollution. Hence, the interest on the geochemistry of groundwater is increasing as a result of a large number of groundwater pollution cases resulting from incidents like underground liquid waste storage, accidental contaminant of groundwater bodies by lethal substances, leakages from sanitary landfills, ponds and lagoons, as well as leaching of animal wastes, fertilizers and pesticides from agricultural soils (Adekunle, et. al., 2007; Olayinka and Alo, 2004; Ikem et. al., 2002).
Generally, the results of the chemical analysis of a large number of samples usually yield a clumsy mass of data where quick visual comparison is difficult (Asiwaju-Bello and Oladeji, 2001). Easily comprehensible techniques of analyzing these data are the graphical methods. Examples of these are the spatial variational maps, bar charts, circular diagrams, and Stiff diagrams. Others include trilinear diagrams developed by Piper (1953), with similar design by Hill (1940), and the semi-logarithmic diagram developed by Scholler (1962). Consequently, it is apparent that the combination of the different types of graphical techniques will be useful in extracting obscured facts from water analytical results.
Reports on geochemical characteristics of groundwater within some parts of the basement (Ako et al., 1990; Malomo et al., 1990; Gbadebo, 2012), as well as some sedimentary basins of Nigeria (Tijani and Uma, 1998; Ekwere and Ukpong, 1994;
Akujieze et al., 2003), are available in the literature. However, within Lagos State, there have been few published works, with the exception of consultant reports which are not freely accessible. Few authors, e,g, Longe et al., (1986), provided little details on the water quality in the areas within the framework of their hydrogeological study. However, the depth of sampling could not be related to the samples analyzed. Therefore, this paper intends to study water quality depth stratification by integrating statistical enquiry and graphical techniques so as to utilize the advantages of the individual methods in the analyses of the hydrochemical results obtained from the study area. The Piper and Schoeller graphical techniques are favoured in this work because of their ability to reflect differences or similarities among different water samples, and their limited space requirement.
The study area
Lagos State is located along the coastal region of south-western Nigeria, and bounded by longtitudes 3ᵒ00’ – 4ᵒ15’ E and latitudes 6ᵒ15’ – 6ᵒ45’ N (Figure 1). The area of investigation is generally low-lying with several points virtually at the sea level, and is therefore prone to flooding. Jones and Hockey (1964) recognized three distinct topographical forms which include the northern uplands, the Ewekoro depression and the low-lying marshy belt at the southern part. The area is drained by four major river systems, namely Ogun in the centre, Ona and Osun in the east, and the Yewa in the west.
Two climatic seasons exist in the area. These are the dry season, spanning from November to March, and wet season, which extends from April to October, with an
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average annual rainfall of 1700 mm (Akintola, 1986). The rainfall serves as a major source of groundwater replenishment in the area.
Figure 1: Description of the study area Geology and Hydrogeology
The geological map of the study area is presented in Figure 2. The area forms the eastern part of the regional Dahomey basin, and extends from Accra in Ghana to Benin hinge line in Nigeria, where it is separated from the Niger Delta by the Okitipupa ridge.
The basal portion of Dahomey basin consists essentially of sandstones and gravels devoid of fossils. This is overlain by marine shales, sandstones and limestones of Albian to Santonian ages. Generally, the geosequence of the Dahomey basin covers Pre-Cambrian to Recent. The Recent sediments occur as alluvial deposits along the major rivers and coastal belts. It consist essentially of unconsolidated coarse to poorly sorted sands, clay lenses and mud, with presence of pyritic fragments and traces of lignites in sediments within the area.
The aquifers within this horizon are unconfined (Asiwaju-Bello and Oladeji, 2001). The Coastal Plains Sands consist of poorly sorted clays with traces of lignites and pyritic fragments and thicknesses increasing from north to south. Kampsax and Sshwed (1977) subdivided the Coastal Plains Sands into the upper and lower portions separated by impermeable argillaceous materials. Details about the geology of the basin can be found in reports by Jones and Hockey (1964), Omatsola and Adegoke (1981), and Fayose (1970).
42 Figure 2: Geology of the study area Methodology
This study utilizes geochemical results of groundwater samples obtained from the study area (see Figure 1). The analyses include the following physical parameters namely, the temperature, conductivity, pH, and colour. The major ionic concentrations included in the analyses are Ca2+, Mg2+, Na+, K+, SO42-, HCO3-, Cl-, total Fe, Si, NO3-, and NH4+
. The physical parameters which include temperature, conductivity, and the pH were measured on the field using mercury thermometer, HANNA pH meter, and Mettler-Toledo EC/TDS meter, respectively. The cations and the anions were determined using Perkin-Elmer 305B model of atomic adsorption spectrophotometer and titration methods (PerkinElmer, 1996), respectively. The hydrochemical data were checked for validity via ionic balance and the data that fall outside the acceptable percentage error of 5 % (Freeze and Cherry, 1979) were discarded.
The data were sorted out based on the area and depth stratification. The study area was subdivided into the northern, the central and the southern parts. The geographical areas that constitute the northern part in this study are Ikeja, Ipaja, Shasha, Ilupeju, Ikorodu, Itoikin, Eredo, and Epe. The central part of the study area is occupied by Igando, Oshodi, Mushin, Isolo, Shomolu, Badore, Lakowe, and Bariga. Also, the southern part consist of Festac, Ijanikin, Apapa, Ojoo, Amuwo, Lekki, Kirikiri, Akodo, Victoria Island, Ikoyi and Badagry. The depth stratification range used for each of the area based on the available data on the depths of completed boreholes are <100 m, 100 – 200 m, and > 300 m, and these hereafter are referred to as the first, second and third horizons, respectively.
The statistical parameters used for the descriptions of the hydrochemical data include the mean, measure of central tendencies, degree of dispersions and correlation.
These helped to reduce the mass of data to a smaller representative collection that
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adequately summarizes the entire set of data, measure consistencies and aid in its comprehension. The water samples were classified in terms of the water types, based on the concept of hydrochemical facies, and correlated using integrated graphical techniques developed by Piper (1953) and Schoeller (1962). Possible mixtures of water were tested by observations within the three fields of the trilinear diagrams, and by the application of the graphical-algebraic criteria suggested by Piper (1953). Comparisons were made between the ionic ratios from various water sources in order to establish the origin of anomalous chemical concentrations. Finally, the suitability of the waters for irrigation purposes was tested using Sodium Adsorption Ratio (SAR) formula proposed by the US Salinity Laboratory (1954).
Results and discussions
The arithmetic mean, range of values, standard deviations and coefficient of variations for the specific electrical conductivity (SEC), temperature, pH, colour, total dissolved solids (TDS) and the major ionic concentrations of the samples obtained from the northern, central and the southern parts of the study area are presented in Tables 1 – 3, respectively. The tables indicate important similarities and differences between the various depth horizons. In all the horizons within the study area, the values of coefficient of variations are considered to be low, and therefore suggest a high degree of consistencies in the hydrochemical data obtained from the study area. Also, in all the horizons, the hydrochemical data indicate a good correlation between the SEC and TDS, with the average ratio of the latter to the former being 0.6. The concentration of other ions in the samples correlated favourably well with the observed trends of the TDS and SEC. The highest values of the TDS are observed at the central area, probably due to more intense anthropogenic activities. The temperature shows an increase with depth and also increases from the north towards the south. The pH value is highest in the north and decreases towards the south. The relatively high pH in the north is perhaps due to the presence of pyritic fragments and traces of lignites in sediments within the area. The colour indicator measured on the Hazen unit increases towards the south which suggest increasing turbidity.
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Table 1: Analytical results of depth stratification of water quality for the northern area
Parameter
Northern Sub area
First Horizon Second Horizon Third Horizon
Ῡ Yn –
Y1 σ n σ1 Ῡ Yn –
Y1 σ n σ1 Ῡ Yn –
Y1 σ n σ1
Conductivity (µS/cm) 150 0 0 1 0 51 53 20 5 0.4 360 156 78 2 0.4
Temperature (ᵒC) - - - - - 29 2.4 0.8 5 0 30 22 10 3 0.2
pH 5.5 0 0 1 0 5.3 0.9 0.4 5 0.1 6.1 0.7 1.3 2 0.2
Colour 1 0 0 1 0 1 1 0.6 5 0.6 7 10 5 2 0.7
TDS (mgL-1) 101 0 0 1 0 34 44 16 5 0.5 249 339 142 3 0.6
Ca2+ (mgL-1) 14 0 0 1 0 5.1 4.9 1.7 5 0.3 31 58 24 3 0.8
Mg2+ (mgL-1) 1.2 0 0 1 0 0.4 0.3 0.1 4 0.3 4.5 8.3 3.4 3 0.8
Na+ (mgL-1) 11 0 0 1 0 5.1 6.3 2.1 5 0.4 44 53 22 3 0.5
K+ (mgL-1) 3.8 0 0 1 0 1.3 4 1.5 5 1.2 15 23 9.8 3 0.6
Total Fe (mgL-1) - - - - - 0.2 0.5 0.2 3 1 3.2 3.4 1.6 3 0.5
Si (mgL-1) 8 0 0 1 0 8 17 6.9 4 0.9 21 35 15 3 0.7
Cl- (mgL-1) 22 0 0 1 0 8.8 14 4.7 5 0.5 15 11 4.5 3 0.3
HCO3
(mgL-1) 9.6 0 0 1 0 8.1 12 4 5 0.5 183 255 111 3 0.6
SO4
(mgL-1) 5 0 0 1 0 2.3 0.5 0.3 5 0.1 3 2 1 2 0.3
NO3- (mgL-1) 33 0 0 1 0 11 13 5.2 4 0.5 5.6 7.7 3.2 3 0.6 NH4+ (mgL-1) 0.5 0 0 1 0 0.5 0.1 0.1 4 0.2 1.6 2.6 1.1 3 0.7
Ῡ: Arithmetic mean; Yn – Y1: Data Range; σ: Standard deviation; n: Number of data; σ1: Coefficient of deviation.
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Table 2: Analytical results of depth stratification of water quality for the central area
Parameter
Central Sub area
First Horizon Second Horizon Third Horizon
Ῡ Yn – Y1 σ n σ1 Ῡ Yn – Y1 σ n σ1 Ῡ Yn – Y1 σ n σ1
Table 3: Analytical results of depth stratification of water quality for the southern area
Parameter
Southern Sub area
First Horizon Second Horizon Third Horizon
Ῡ Yn – Y1 σ n σ1 Ῡ Yn – Y1 σ n σ1 Ῡ Yn – Y1 σ n σ1
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Within the third horizons in the north and central areas, HCO3- is the dominant anion, followed by Cl- ions. The average ratio of the latter to the former within these horizons is approximately 10. The Cl- and NO3
-ions dominate the first horizons of the north and the central parts. The relative concentrations of the alkaline and alkali earth metals within the horizons are similar in these areas. The patterns of the chemical characteristics of the water samples from all the horizons as shown by the Schoeller’s semi-logarithmic diagrams are presented in Figure 3. Generally, in all the areas and the horizons, the pH values of 5.3 - 6.7 obtained indicate slight acidity when compared with the WHO (2011) recommended guideline of 6.5 – 9.2. However, the pH values observed in this study agrees with the work of Edet, et. al., 2011, where it was observed that most groundwater samples in Nigeria have pH values of between 4.86 and 7.05. The average values obtained for TDS (34 – 305 mgL-1), Mg2+ (0.2 – 4.5 mgL-1), Na+ (5.1 – 50 mgL-1), Cl- (8.8 – 65 mgL-1) and SO4
(2.0 – 13 mgL-1) fall below the recommended highest desirable limits of 500, 30, 100, 200, and 200 mgL-1, respectively.
Also, Si, HCO3-, Ca2+, and NO3- ions fall largely below the W.H.O. (2011) recommended limits. The average total iron concentration within the study area is generally high. In the south, high iron concentration was observed in all the three horizons with average values of 1.5, 12, and 3.7 mgL-1, in the first, second and third horizons, respectively. These iron concentrations exceed the WHO (2011) maximum permissible level of 1.0 mgL-1. The suitability of the waters for irrigation purpose is dependent on the types of plant, soil and the climate. The replacement of Ca and Mg ions by sodium ions which could cause reduction in the soil permeability and hardening of the soils is measured by the SAR formula (US Salinity Laboratory, 1954), given as:
√ (1)
The SAR values have been computed for the waters in all the horizons of the study area and the results are presented in Table 4, and the range of the SAR values is 3.08 – 14.00. According to the classification presented by Etu-Efeotor (1981), all the water samples in the study area are described as ‘Excellent to Good’, and therefore are suitable for irrigation purposes.
On the Piper’s trilinear diagram (Figure 4), the water samples are categorized based on the classification proposed by Piper (1953). In the northern area, samples from the second horizon plotted at sub-area 4 of the diamond-shaped field, and this suggests that the strong acids exceed the weak acids.
Table 4: Values obtained for the SAR
Horizon SAR Values Water Class (after
Etu-Efeotor, 1981) North Central South
First 3.98 14 7.8 Excellent - Good
Second 3.08 4.87 6.06 Excellent
Third 10.46 11.04 8.5 Excellent - Good
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Figure 3: Scholler semi-logarithmic diagram
The relatively high pH value earlier observed in this horizon agrees with this deduction. The third horizon in the north falls into the sub-area 3 of the diamond field, and this suggests the prevalence of weak acids as compared to strong acids. The first and the second horizons in the central part fall within the sub-area 7 of the field, and this suggests primary salinity where the chemical properties of the waters are dominated by the alkalis and strong acids, typifying the oceanic waters and brine.
In order to demonstrate conclusively that certain water is a quantitative mixture of two other waters, Piper (1953) suggested that the apparent mixture must plot on a straight line between the plotting of its two inferred components in each of the three fields of the trilinear diagram. In addition, the following equations should also be satisfied:
( ⁄ ) ⁄ (2)
( ⁄ ) ⁄ (3)
( ) ( ⁄ ) (4)
⁄( ) (5)
⁄( ) (6)
where
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a, b: distances measured on the diamond field of the Piper’s diagram
: concentrations in the samples a and b, and mixture m, respectively.
: Proportionate volume of samples a and b in the mixture m.
Therefore, with reference to Figure 4, the samples which show a degree of alignment with the plotting of the inferred components of rain water and sea water concentrations are the samples from the first and second horizons in the central parts. The compliance of these samples with equations 2 – 6 was further tested and the results indicate a percentage difference of 50.5 and 25, respectively, between the empirically determined and computed concentrations. This suggests that the salinity of the two samples earlier observed is not due to the sea water intrusion. Furthermore, Table 5 presents the ionic ratios of the major chemical parameters of the samples obtained from the central and southern areas, as well as similar ratios for the sea water constituents in the same area. Similarities exist in the values of the ionic ratios between the two sea water samples obtained from Akodo and Bar Beaches. However, there are large discrepancies between the set of the two sea water samples and those obtained from the horizons (Table 5). This further supports the deduction that the observed salinity is not due to sea water intrusions.
The summary of the hydrofacies classification of the water samples obtained from the study area in all the horizons is presented in Table 6. With the exception of the third horizons in the north and central parts that are dominated by the HCO3- facies type, all the remaining horizons within the study area are dominated by Cl- facies type. In the north and the central parts, transformation of the water facies type from Cl--SO4
type at the first horizon through Cl--SO42--HCO3- type at the second, and to HCO3--Cl--SO42- at the third horizon was observed. In the southern area, the observed water facies is Cl-- SO4
2-HCO3
in all the horizons.
The chemical composition of dissolved salts can be used to characterize the origin of salinity in water samples. Shale consists essentially of clay minerals that have capacity for ionic exchange. Also, the high porous aquifers in the horizons where relatively high chloride values are observed are likely to be composed of an appreciable percentage of colloidal sizes particles which have the capability to exchange ion constituents that are adsorbed on the particle surfaces. Hence, it may be concluded that the major sources of these relatively high chloride values observed are the ionic exchange and dissolution of minerals within the strata. Other probable sources could be the presence of the remnants of marine water trapped through ancient transgression episodes, with subsequent chemical alteration by interaction with the aquifer materials and differential dilution caused by the infiltrating fresh water, though this is not substantiated in this study.
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Figure 4: Trilinear Piper plot for the chemical characters of the water samples.
Table 5: Calculated ionic ratio of some of the chemical parameters.
Sample source ⁄ ⁄ ( )
⁄ ⁄
First Horizon (Central) 6.15 0.77 0.89 0.37
Second Horizon (Central) 3.58 0.35 0.45 0.95
First Horizon (South) 4.21 1.29 1.6 3.29
Second Horizon (South) 3.97 0.99 1.24 2.99
Third Horizon (South) 5.4 1.3 1.54 2.9
Akodo sea water 24.57 0.52 0.56 0.01
Bar Beach sea water 32.12 0.55 0.57 0.01
Table 6: Hydrochemical facies classification of waters with the horizons
Horizon North Central South
Anion Cation Total Anion Cation Total Anion Cation Total
First Cl NDT* Cl-SO4 Cl Na-K Cl-SO4 NDT* Na-K Cl-SO4-HCO3
Second Cl NDT* Cl-SO4-HCO3 Cl Na-K Cl-SO4-HCO3 NDT* Na-K Cl-SO4-HCO3 Third HCO3 Na-K HCO3-Cl-SO4 HCO3 Na-K HCO3-Cl-SO4 Cl Na-K Cl-SO4-HCO3
*: No dominant type
Conclusion
Two hydrochemical regimes are revealed by the analyses carried out in this study.
The first regime is found within the three horizons in the south, as well as in the two
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upper horizons in the north and the central parts. This regime is dominated by Na+-Cl -facies type, with relatively high iron concentrations. The second hydrochemical regime, dominated by Na+-HCO3
facies type, is found within the third horizon of the north and central parts of the study area. The results of the analyses of the relatively high chloride values observed within the first hydrochemical regime support a non-sea water intrusion source but rather, believed to be due to ionic exchange, ionic precipitation and dissolution of the host geologic materials. Other probable sources that are not substantiated in this work could be the presence of water of ancient origin, trapped by marine sediments or concentrated by shale membrane. The spatial variation observed may be due to differential dilution by the infiltrating precipitates. The pH values indicate high acidity in the northern part, and this may be due to the presence of pyritic fragments in this sub-area. However, within the central and the southern parts, the pH values fall within the
facies type, is found within the third horizon of the north and central parts of the study area. The results of the analyses of the relatively high chloride values observed within the first hydrochemical regime support a non-sea water intrusion source but rather, believed to be due to ionic exchange, ionic precipitation and dissolution of the host geologic materials. Other probable sources that are not substantiated in this work could be the presence of water of ancient origin, trapped by marine sediments or concentrated by shale membrane. The spatial variation observed may be due to differential dilution by the infiltrating precipitates. The pH values indicate high acidity in the northern part, and this may be due to the presence of pyritic fragments in this sub-area. However, within the central and the southern parts, the pH values fall within the